In early sea urchin development, an important experiment involves isolating individual blastomeres from a 4-celled embryo. Each isolated blastomere can independently develop into a complete pluteus larva, demonstrating remarkable developmental plasticity. This phenomenon is a classic example of conditional specification in embryonic development.

Conditional specification means that the fate of a cell is not rigidly predetermined but depends partly on its environment and interactions with other cells. When isolated, these blastomeres compensate for the absence of their neighbors by regulating gene expression and developmental programs to form a whole larva.

This stands in contrast to autonomous specification, where cells have fixed fates determined by internal factors, making it impossible for isolated cells to form complete structures.

Mosaic development refers to embryos in which each cell’s fate is autonomous, and removal or isolation of cells results in missing parts, unlike in this sea urchin example.

Determination is a stage where cells irreversibly commit to a fate, but it does not explain the flexibility observed in isolated blastomeres.

Thus, the observed developmental outcome in the sea urchin blastomere isolation experiment strongly supports:

(2) Conditional specification